Electronic phase control circuit



Aprll 21 1959 J. D. SALISBURY ET A 2,883,536

' ELECTRONIC PHASE CONTROL CIRCUIT Filed March 5, 1958 -|f '72T /3T I I l ,/4 I /8 22 1/ A k l- 1 I l RE PHASE RE AMPLIFIER DETECTOR AMPLIFIER I III/ .f 1 23 I 27 I AMPLI IER I6 25 2 f I MASTER BUFFER OSCILLATOR A PLIFIER DIRECT CURRENT VOLTAGE INVENTORS. JOHN D. SALISBURY BY WALTER W KLEIN CALVIN F HANSEN ATTORNEY.

ELECTRONIC PHASE CONTROL CIRCUIT John D. Salisbury, Livermore, and Walter W. Klein, Fullerton, Calif, and Calvin F. Hansen, Bountiful, Utah, assignors to the United States of America as represented by the United States Atomic Energy Commission Application March 5, 1958, Serial No. 719,462

5 Claims. (Cl. 250-27) The present invention relates to the phase control of radio frequency energy, and more particularly, to an electronic circuit for controlling the phase of radio frequency energy applied to a multi-cavity linear accelerator.

Present day multi-cavity linear accelerators are widely used for accelerating charged particles to extremely high velocities with an attendant increase in particle energy proportional to a squared function of the velocity. The resulting high energy particles might be used, for example, to bombard a target and thus induce various nuclear reactions with the target material. Such reactions are generally accompanied by the release of X-rays, neutrons, and/or other constituents of atomic nuclei useful for serving manifold utilitarian purposes.

For a detailed description of one type of multi-cavity linear accelerator, reference may be had to the copending application of Hester et al., Serial No. 502,251, filed April 18, 1955, now Patent No. 2,860,279. Basically, in such accelerator, as Well as other multi-cavity accelerators, several axially communicating resonant cavities are excited with radio frequency energy to a predetermined resonant state whereby a standing wave field is established in each cavity. A charged particle, e.g., a proton, deuteron, or the like, injected into the first cavity in proper phase relationship to the radio frequency standing wave field therein is accelerated by such field to then leave the first cavity and enter the second cavity in an accelerated state. If, upon entering the second cavity the standing wave field therein is cycling in a direction favorable to that of the particle, the particle will be further accelerated. Similarly, the particle will be accelerated in each successive cavity of the accelerator provided the phase relationship between the radio frequency energization applied to adjacent cavities is properly maintained so as to continuously eifect particle acceleration. For example, in order that a particle accelerated during the positive half cycle of energy in one cavity, be accelerated in the next successive cavity, the energy in the latter cavity must undergo a positive half cycle excursion substantially simultaneously with the entry of the particle thereinto. In other words, a 180 phase relationship must be maintained between the energies in the successive cavities.

Various systems have been utilized to control the phase of the radio frequency excitation energies applied to suecessive cavities of multi-cavity linear accelerators. A predetermined phase relationship between cavity energies is often maintained, for example, by means of servomechanisms wherein departures from said predetermined phase relationship are continuously detected and compensated by altering the phase of the excitation energy applied to one cavity with respect to that applied to the other by an amount equal and oposite to the detected departure. Systems of the foregoing type have been largely electromechanical in nature, however, and accordingly are limited in response time by the inertial effects of the mechanical elements employed. At energization frequencies of the order of 50 me. as employed in many present day multi-cavity accelerators, electromechanical control systems are inherently too slow acting to adequately compensate departures from the phase relationship between excitation energizations requisite to optimum particle acceleration.

ted States Patent 0 2,883,536 Patented Apr. 21, 1959 "Ice The present invention provides a completely electronic phase control circuit for multi-cavity linear accelerators which is thereafter not limited in time of response by mechanical components and hence overcomes many of the limitations and disadvantages of electromechanical control systems. The invention may be advantageously used to continuously and accurately maintain a predetermined phase relationship between several oscillating energies at frequencies including the very high frequency (VHF) band for multi-cavity accelerator applications, as well as other control applications, by virtue of novel electronic phase delay and advancement means employed therein.

It is therefore an object of the present invention to provide an electronic control circuit for maintaining predetermined phase relationships between various radio frequency voltages and currents in multi-cavity linear accelerators.

Another object of the invention is the provision of a phase control system having an extremely fast response time.

Still another object of the invention is to provide a circuit of the above-noted character which is capable of reliable operation at very high frequencies as high as 50 me. and higher.

It is a further object of this invention to provide a phase control circuit including electronic phase retardand advancing means comprising a lumped constant delay network wherein series inductors are coupled by means of voltage-sensitive shunt capacitors.

Additional objects and advantages of the invention will become apparent from the following description and claims considered together with the accompanying drawing, of which:

Figure 1 is a schematic block diagram of a preferred embodiment of the invention as employed to control the phase of radio frequency energy applied to adjacent cavities of a multi-cavity linear accelerator; and

Figure 2 is a graphical illustration of the phase shift to capacitor DC. bias voltage characteristic of electronic phase delay means of this embodiment.

Considering now the present invention in some detail and referring to the illustrated form thereof in the drawing, there is provided a phase control circuit 11 for maintaining a predetermined phase relationship between radio frequency excitation energies applied to first and second resonant cavities 12, 13, respectively, of a multi-cavity linear accelerator 14. More particularly, radio frequency energy of a very high frequency, e.g., 50 me. is generated by means of a radio frequency energy source such as a master oscillator 16 coupled directly to a first radio frequency amplifier 17 the output of which is in turn coupled, as by means of a coupling loop 18, to cavity 12. The dimensions of cavity 12 are chosen such that the cavity is resonant at the frequency of the radio frequency energy generated by master oscillator 16. Moreover, such radio frequency energy is amplified in amplifier 17 to proportions commensurate with the acceleration of charged particles whereby a radio frequency standing wave particle accelerating field is established within cavity 12.

Master oscillator 16 is additionally coupled through a phase delay circuit 19 in accordance with the present invention to a second radio frequency amplifier 21. The output of amplifier 21 is in turn coupled in energizing relationship to the second cavity 13 as by means of a coupling loop 22. As in the case of cavity 12 hereinbefore described, cavity 13 is designed to be resonant at the frequency of radio frequency energy generated by master oscillator 16 and amplifier 21 is effective in amplifying the energy to establish a radio frequency standing wave particle accelerating field within cavity 13. Phase delay circuit 19, however, shifts the phase of the radio frequency energy applied thereto from master oscillator 16 by a predetermined amount in a manner hereinafter described whereby the standing wave accelerating field established within the cavity 13 is shifted in phase relative to the field established within cavity 12 by substantially said predetermined amount. It is to be appreciated that the predetermined phase relationship between respective cavity oscillations is selected so as to effect continued acceleration of charged particles leaving the first cavity 12 and entering the second cavity 13, e.g., in many multi-cavity accelerators the phase relationship might be 180. Moreover, any departure from the desired predetermined phase relationship, as commonly occurs in practice due to inherent drifts in the resonant frequency of cavities, is herein continuously detected and converted to a proportional bipolar D.C. error voltage indicative of the amount and direction of such departure, preferably by means of a phase detector 23 coupled between cavities 12, 13. For a detailed description of a suitable phase detector, reference may be had to a copending application of Dean 0. Kippenhan, Serial No. 595,033, filed June 29, 1956, which phase detector may be adjusted to detect departures from any one of a Wide range of phase relationships. The error voltage output of phase detector 23 is coupled to phase delay circuit 19 and in accordance with the salient aspects of the invention subsequently described, the phase delay circuit in response to the error voltage functions in a novel manner to rapidly alter the phase shift introduced to the excitation energy applied to cavity 13 by an amount nearly equal and opposite to the detected departure from the desired phase relationship between cavities 12, 13. Phase delay circuit 19 thus functions to at all times maintain the phase of the radio frequency excitation of cavity 13 relative to the excitation of cavity 12 in the predetermined relationship within close tolerances whereby optimum acceleration of charged particles in the cavities is attained.

Considering now the phase delay circuit 19 of previous mention in detail, particularly with reference to the salient aspects of same, there is provided a novel electronic phase delay line 24 coupled in series with the output of master oscillator 16 as by means of a bulfer amplifier 25 and connected at its output to the second radio frequency amplifier 21. Delay line 24 in accordance with the salient aspects of the present invention is provided as a lumped constant ladder network comprising a plurality of series inductance coils 26 and shunt voltage sensitive capacitors 27 connected between the junctures of the coils and ground. More particularly, voltage sensitive capacitors 27 include a dielectric, such as barium titanate, whose dielectric constant varies in inverse relation to the D.C. voltage impressed thereacross whereby the capacitance of the capacitors similarly varies inversely with respect to the impressed voltage. The phase delay, 0, of a delay line such as delay line 24 is expressed by: =21rf /IT where:

f=frequency of radio frequency voltage applied to the line L=total inductance of line C=total capacitance of line Thus the phase delay, 0, varies directly with the square root of the instantaneous total capacitance, C, of voltage sensitive capacitors 27 and therefore by an inverse function of the D.C. voltage impressed across the line. More particularly, the inverse function of the variation of phase delay, 0, of delay line 24 with respect to the D.C. voltage across the line is generally as depicted by the typical characteristic curve of Fig. 2. As shown therein, the phase delay varies in an inverse relation to the D.C. voltage. A phase delay, 4 corresponding to the desired predetermined phase relationship to be established between radio frequency energies applied to cavities 12, 13 plus a slight error to facilitate operation of delay circuit 19 at all times (as is common practice in the closed loop servomechanism art) may be introduced to the radio frequency energy applied to cavity 13 through delay line 24, by impressing a D.C. bias voltage, E, across the line of such a magnitude as to produce the desired delay, p, plus a slight error in accordance with the characteristic curve.

In order to facilitate the foregoing, a D.C. amplifier 28 as shown in Fig. 1 is connected with its output in parallel with voltage sensitive capacitors 27 of delay line 24 and preferably at the center of the line. Amplifier 28 is arranged in a conventional manner as by means of appropriate grid bias means to produce the output bias voltage E corresponding to the desired delay plus a slight error with zero voltage applied to the input of the amplifier. Moreover, the input of amplifier 23 is connected to the error voltage output of phase detector 23 whereby such error voltage correspondingly varies the output voltage of the amplifier in direct relationship to such error voltage. Therefore, error voltage generated in phase detector 23 in response to departures from the desired predetermined phase relationship between cavities 12, 13, a value of which error voltage is at all times generated due to the slight phase error deliberately introduced into the excitation energy, is effective in correspondingly changing the output voltage of amplifier 28 from bias voltage, E, to values deviating in magnitude and direction from such bias voltage in direct proportion to the phase departures. The phase delay of delay line 24 is then responsively inversely varied to approach delays deviating from the desired delay, by amounts inversely proportional to the deviations in amplifier output voltage from bias voltage, E, the overall result being thus to continuously alter the phase delay of the radio frequency energy applied to cavity 13 by amounts which re at all times opposite and closely approaching the magnitude of the detected departure from the desired predetermined phase relationship between cavities 12, 13.

The foregoing may be better understood through consideration of the following illustrative examples wherein the phase relationship at between radio frequency energies applied to cavities 12, 13 is requisite to optimum acceleration of charged particles in linear accelerator 14 and such relationship 5 plus a slight error, +A is established with the D.C. bias voltage, E, applied to delay line 24 from D.C. amplifier 28 as previously described. Under the above conditions, it will be appreciated that several situations may exist as regards the actual phase relationship between the radio frequency energies applied to cavities 12, 13 as compared to the requisite predetermined relationship, (11, which is to be maintained. Firstly, the actual phase relationship may be equal to the desired phase delay, 5, and deliberate phase delay, +A as corresponds to the bias voltage, E, applied to delay line 24 while conditions of zero drift exist between the cavities 12, 13. Phase detector 23 applies a positive D.C. error voltage, directly proportional to the deliberate error, +A to the input of D.C. amplifier 28. in response to such error voltage, the output voltage of amplifier 28 is increased by a proportional amount, +AE0, such that a resultant D.C. bias voltage, E+AE is applied to delay line 24. Moreover, the circuit constants of phase detector 23 and delay line 24 are selected such that the abovenoted voltage increase, AE produces a corresponding deviation in the phase delay, 0, of delay line 24 which is equal to Arfi Therefore, as noted in Fig. 2, the bias voltage E-l-AE applied to delay line 24 effects the introduction of a phase delay (+AQA to the radio frequency energy applied through the line to cavity 13. It is thus to be noted that the phase delay circuit 19 tends to compensate for the deliberate error, +A such that the phase of the energy applied to cavity 13 at all times approaches the desired delay, 3.

Secondly, the resonant frequency of cavities 12, 13 may drift, such that in addition to the deliberate phase error, Aqb there is a drift error or departure, M, in the actual phase relationship from the desired relationship 4;. Moreover, departures, Aqi, may be in directions above or below the desired relationship whereby the resulting actual relationships are, respectively, +A+A and -A+Ao. In the former case where the actual relationship is equal to +A+A phase detector 23 detects the overall departure, A +A, from the desired relationship, and rapidly applies a directly proportional positive error voltage to the input of D.C. amplifier 28 in a manner which follows that described above. In response to such error voltage, the output voltage of amplifier 28 is increased by a proportional amount, +AE where AE is composed of a compent AE due to the deliberate error A420 and a component, AE, due to the drift departure A. A resultant D.C. bias voltage, E-l-AE =E+AE,,+AE, is consequently responsively applied to delay line 24. Therefore, as noted in Fig. 2, the bias voltage E-l-AE, immediately produces a corresponding delay of substantially -Ao-A in the radio frequency energy applied to cavity 13. Thus the induced phase delay is rapidly decreased by an amount approaching the value, Ae -l-Aep, by which the actual phase delay between the cavity energizations departs above the desired value p. The actual phase relationship is thus regulated to approach the desired relationship, qi.

Considering now the latter case of phase departure below the desired relationship i.e., where the actual phase relationship is +A --A, the operation of the phase control circuit follows from that just described in relation to phase departure above the desired phase relationship. Phase detector 23 applies a negative D.C. voltage to the input of D.C. amplifier 23, which voltage is proportional to the phase departure, A0-A. The bias voltage impressed across delay line 24 from amplifier 23 is responsively decreased by an amount,

to the value, EAE The corresponding phase delay elfected by the delay line as depicted on the characteristic curve therefore, is consequently +A-A whereby the actual phase delay of the radio frequency energy applied to cavity 13 relative to that applied to cavity 12 is rapidly increased by substantially the value, A-A to thereby approach the desired phase delay It is therefore apparent from the foregoing, that the phase delay circuit 19 including the novel electronic delay line 24 will tend to continuously maintain any desired predetermined phase relationship between radio frequency energies, even in the VHF portion of the frequency spectrum, as may be applied to adjacent cavities of multicavity linear accelerators, as well as other apparatus, resulting in optimum particle acceleration and various other beneficial results. When the phase of radio frequency energy applied to a quantity of cavities greater than two is to be controlled, it will be appreciated that a phase control circuit 11 of the type herein described must be connected between each successive cavity and the preceding cavity with the master oscillator being commonly connected in energizing relationship to each circuit.

While the present invention has been hereinbefore described with respect to but a single preferred embodiment, it will be apparent that numerous modifications and variations are possible within the spirit and scope of the invention, and thus it is not intended to limit the invention except by the terms of the following claims.

What is claimed is:

1. In a radio frequency phase control circuit for a multi-cavity linear accelerator having at least first and second axially communicating resonant cavities, the combination comprising a radio frequency energy source coupled to said first cavity for energizing same with radio frequency energy at resonance, an electronic phase delay line coupled between said source and said second cavity for energizing same with radio frequency energy at resonance and with a predetermined phase delay relative to the energization of said first cavity to produce optimum acceleration of charged particles in said accelerator, said delay line producing phase delays in inverse relationship to D.C. bias voltage applied thereto, bias means for generating D.C. bias voltage in direct proportion to D.C. voltage applied to the input of said bias means and connected to said delay line, said bias means generating a bias voltage corresponding to said predetermined phase delay plus a slight phase error with zero voltage applied to its input, and phase detector means having its input coupled between said first and second cavities and output connected to the input of said bias means for generating a directly proportional bipolar D.C. error voltage in response to departures from said predetermined phase delay between the radio frequency energies coupled to said first and second cavities whereby the phase delay of said delay line is continuously regulated in inverse relationship to said D.C. error voltage and said departures tend to be at all times compensated.

2. A phase control circuit as defined by claim 1, further defined by said electronic delay line comprising a lumped constant ladder network having a plurality of inductance coils connected in series and a plurality of voltage sensitive capacitors correspondingly connected in shunt relationship between the junctures of the coils and ground, said capacitors having a dielectric with a dielectric constant that varies in inverse relation to D.C. voltage impressed thereacross.

3. A phase control circuit as defined by claim 2 wherein the dielectric of said capacitors is barium titanate.

4. A radio frequency phase control circuit for establishing a constant predetermined phase delay between radio frequency energies applied to first and second resonant cavities comprising in combination, a master oscillator for producing radio frequency energy at the resonant frequency of said cavities, a first radio frequency amplifier connected to said oscillator and coupled to said first cavity, a buflfer amplifier connected to said master oscillator, a lumped constant ladder delay network formed of a plurality of series connected inductance coils and a plurality of voltage sensitive capacitors correspondingly connected in shunt relationship between the junctures of said coils and ground, said capacitors having a dielectric with dielectric constant that varies in inverse relation to D.C. voltage impressed thereacross, said series connected inductance coils coupled in series with said buffer amplifier, a second radio frequency amplifier coupled in series with said delay network and coupled to said second cavity, a D.C. amplifier coupled in parallel with the voltage sensitive capacitors of said delay network and applying a D.C. bias voltage to said delay network producing said predetermined phase delay plus a slight phase error in response to zero voltage impressed at the input of said D.C. amplifier, and a phase detector having its input coupled between said first and second cavities and output connected to the input of said D.C. amplifier for applying a directly proportional bipolar D.C. error voltage thereto in response to departures from said predetermined phase delay between the radio frequency energies applied to said first and second cavities.

5. A radio frequency phase control circuit as defined by claim 4, further defined by the dielectric of said capacitors being barium titanate.

References Cited in the file of this patent UNITED STATES PATENTS 2,556,978 Pierce June 12, 1951 2,565,231 Hepp Aug. 21, 1951 2,568,250 OBrien Sept. 18, 1951 2,608,654 Street Aug. 26, 1952 2,707,751 Hance May 3, 1955 2,836,759 Colgate May 27, 1958 

